Oxygen-doped n-type gallium nitride freestanding single crystal substrate

ABSTRACT

Oxygen can be doped into a gallium nitride crystal by preparing a non-C-plane gallium nitride seed crystal, supplying material gases including gallium, nitrogen and oxygen to the non-C-plane gallium nitride seed crystal, growing a non-C-plane gallium nitride crystal on the non-C-plane gallium nitride seed crystal and allowing oxygen to infiltrating via a non-C-plane surface to the growing gallium nitride crystal. 
     Otherwise, oxygen can be doped into a gallium nitride crystal by preparing a C-plane gallium nitride seed crystal or a three-rotationally symmetric plane foreign material seed crystal, supplying material gases including gallium, nitrogen and oxygen to the C-plane gallium nitride seed crystal or the three-rotationally symmetric foreign seed crystal, growing a faceted C-plane gallium nitride crystal having facets of non-C-planes on the seed crystal, maintaining the facets on the C-plane gallium nitride crystal and allowing oxygen to infiltrating via the non-C-plane facets to the gallium nitride crystal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an oxygen doping method into a gallium nitridecrystal and an oxygen-doped n-type gallium nitride single crystalsubstrate for producing light emitting diodes (LEDs), laser diodes (LDs)or other electronic devices of groups 3 and 5 nitride semiconductors.Nitride semiconductors means GaN, InGaN, InAlGaN and so on which aregrown as thin films on a sapphire substrate. An activation layer is aGaInN layer. Other parts are mainly GaN layers. Thus, the light emittingdiodes based upon the nitride semiconductors are represented as GaN-LEDsor InGaN-LEDs which mean the same LEDs.

This application claims the priority of Japanese Patent Application No.2001-113872 filed on Apr. 12, 2001 which is incorporated herein byreference.

2. Description of Related Art

Light emitting devices making use of nitride semiconductors have beenput on the market as blue-light LEDs. At present, all of the availablenitride light emitting devices are made upon sapphire substrates. Anepitaxial wafer is obtained by growing a GaN film, a GaInN film and soforth upon a C-plane single crystal sapphire substrateheteroepitaxially. A unique n-dopant for GaN, AlInGaN, or InGaN thinfilms is silicon (Si). Silicon acts as an n-impurity in GaN by replacinga gallium site. A series of wafer processes produces GaInN-LEDs on theon-sapphire epitaxial wafer. A lattice constant of sapphire (α-Al₂O₃) isdifferent from that of gallium nitride. Despite the large latticemisfit, a sapphire substrate allows gallium nitride to growheteroepitaxially on it. The on-sapphire GaN includes great manydislocations. In spite of the many dislocations, the GaN films onsapphire are stable and endurable.

Sapphire is a crystal of a trigonal symmetry group. C-plane of sapphirehas quasi-three fold rotation symmetry. Gallium nitride belongs tohexagonal symmetry. C-plane of gallium nitride has perfect three-foldrotation symmetry. Since the symmetry groups are different for GaN andsapphire, any other planes than C-plane of sapphire cannot grow a GaNcrystal. Thus, the GaInN-LEDs in use include sets of c-axis grown InGaN,InGaAlN or GaN thin films grown on C-planes of sapphire substrates.

All the GaN or GaInN thin films heteroepitaxially grown on the sapphiresubstrates are C-plane growing crystals. Sapphire substrates cannot makenon-C-plane growing GaN crystals at all. Since sapphire has been aunique seed crystal for growing GaN until recently, it has beenimpossible to make a non-C-plane GaN crystal. At present, all theGaInN-LEDs and GaInN-LDs on the market consist of a pile of C-planegrown GaN, InGaN or AlInGaN thin films grown on C-plane sapphiresubstrates.

Large lattice misfit between sapphire and gallium nitride induces plentyof dislocations in a gallium nitride crystal grown on a sapphiresubstrate. Gallium nitride has rigidity as high as ceramics. Therigidity maintains the framework of crystals for a long time. Inherentdislocations in GaN crystals of LEDs do not enlarge by current injectionunlike GaAs crystals. Since the dislocations do not increase, the GaNcrystals on sapphire do not degrade. In spite of the great manydislocations, GaN-LEDs enjoy a long life time, high reliability and goodreputation.

Sapphire substrates, however, have some drawbacks. Sapphire is a veryrigid, hard crystal. Sapphire lacks cleavage. Sapphire is an insulator.Rigidity, non-cleavage and insulation are weak points of sapphire. Whena plenty of device units have been fabricated upon a sapphire substratewafer by wafer processes, the device-carrying sapphire wafer cannot bedivided by natural cleavage like silicon wafers. The sapphire wafershould be mechanically cut and divided into individual device chips. Themechanical dicing step raises the cost.

The non-cleavage is not a serious obstacle for making LEDs (lightemitting diodes) on sapphire substrates, since an LED has no resonatormirror. In the case of producing LDs (laser diodes) on sapphiresubstrates, the non-cleavage is a fatal drawback. A laser diode (LD)requires two mirrors at both ends of an active (stripe) layer as aresonator for amplifying light by injected current. It is convenient toform resonator mirrors by natural cleavage in a laser diode, becausenatural cleaved planes are endowed with flatness and smoothness.On-sapphire LDs prohibit natural cleavage from making resonator mirrors.Flat, smooth mirrors should be made on both ends of the laser chips by avapor phase etching method, e.g., RIE (reactive ion etching), which is adifficult operation. Mirror-polishing should be done chip by chip afterthe wafer process has finished. Mirror-finishing of the resonators bythe RIE is a main reason raising the cost of manufacturing theon-sapphire GaInN-LDs.

Another drawback results from the fact that sapphire is an insulator.Insulation prevents on-sapphire LEDs and LDs from having an n-electrodeon the bottom. Sapphire forces LEDs and LDs to have extra n-type layersupon an insulating substrate but below an active layer. The n-electrodeis formed by partially etching away a p-layer and the active. layer,revealing the extra n-layer and depositing an n-electrode alloy on then-layer. Both a p-electrode and the n-electrode are formed on the topsurface of the LED or LD. Since electric current flows in the horizontaldirection, the n-layer should have a sufficient thickness. It takes muchtime to eliminate a part of the p-layer and form an ohmic n-electrode onthe revealed n-layer. An increase of the steps and time enhances thecost of the on-sapphire LEDs. Both the n-electrode and the p-electrodeoccupy a wide area on the top of the LED, which raises a necessary areaof the LED. On-sapphire GaInN-LEDs which are prevailing cannot conquerthe above drawbacks yet.

A gallium nitride (GaN) single crystal substrate would be an idealsubstrate which has a probability of solving the drawbacks. Since thinfilms of GaN or GaInN are epitaxially deposited upon a substrate forproducing blue light LEDs and LDs, a GaN bulk single crystal wouldeliminate the problem of lattice misfitting between the deposited filmsand the substrate. If an n-type bulk single crystal GaN substrate can beproduced, an n-electrode can be formed on the bottom of the n-type GaNsubstrate. An allocation of a p-electrode at the top and an n-electrodeat the bottom facilitates to produce LEDs, to bond the LEDs on packages,and to wirebond the LEDs to wiring patterns on the packages. The bottomn-electrode enables an LED to reduce the chip size.

Another advantage results from cleavability of a bulk GaN singlecrystal. A device-produced GaN wafer can be divided into stripe arraysof individual device (LED or LD) chips by natural cleavage. However,cleavage planes (1-100), (01-10) and (−1010) are parallel to three sidesof an equilateral triangle defined upon a C-plane (0001) of GaN. The GaNcrystal has not a square set of cleavage planes but a triangle set ofcleavage planes. Square device (LED or LD) chips are produced by cuttinga device-carrying GaN wafer partially by natural cleavage and partiallyby mechanical dicing.

Furthermore, an LD (laser diode) chip can produce resonator mirrors bynatural cleavage. Replacement of the RIE by the natural cleavage reducesthe cost of making GaInN-type blue light laser diodes (LDs).

However, there is no mineral containing gallium nitride single crystals.No attempt of making a wide, bulk GaN single crystal substrateartificially has succeeded until recently. Since a GaN single crystalsubstrate was inaccessible, it was not possible to make GaInN type LEDsor LDs on a single crystal GaN substrate until recently.

Recently, vapor epitaxial methods which can grow a GaN single crystal ona foreign material substrate have been proposed and improved. Themethods are described as follows.

(1) Metallorganic Chemical Vapor Deposition Method (MOCVD)

The most prevailing method for making GaN crystals is a MetallorganicChemical Vapor Deposition Method (MOCVD). The MOCVD produces a GaNcrystal by placing a sapphire substrate in a cold-wall furnace, heatingthe sapphire substrate, supplying a material gas including TMG(trimethylgallium) and ammonia (NH₃) to the sapphire substrate, andsynthesizing gallium nitride (GaN) from the material gas on thesubstrate. Although a great amount of the material gas is inhaled intothe furnace, only a part of the material gas reacts with each other formaking gallium nitride molecules. Other part of the material gas isdissipated in vain. The MOCVD is suffering from low yield and lowgrowing speed. The MOCVD is favorable for making thin GaN films but isunsuitable for producing a thick GaN crystal layer due to the materialdissipation. Another drawback is possibility of carbon contamination dueto carbon included in the metallorganic gases.

(2) Metallorganic Chloride Method (MOC)

An MOC method produces a GaN crystal by placing a sapphire substrate orGaAs substrate in a hot-wall furnace, supplying TMG (trimethylgallium)and HCl (hydrochloric acid) into the furnace, synthesizing GaCl (galliumchloride) above the substrate, supplying ammonia (NH₃) to the heatedsubstrate, inducing a reaction between NH₃ and GaCl on the substrate,making gallium nitride molecules on the substrate and depositing thegallium nitride molecules on the substrate. Since the MOC method makesonce an intermediate compound GaCl, carbon contamination is alleviatedin comparison with the MOCVD. However, the MOC is not fully immune frompossibility of carbon contamination, since the MOC employstrimethylgallium gas.

(3) Hydride Vapor Phase Epitaxy Method (HVPE)

Unlike the MOCVD or the MOC, an HVPE employs metal Ga monoelement as agallium source. FIG. 1 shows a HVPE apparatus having a hot-wall furnace1. A heater 2 is upholstered around the furnace 1. Gas inlets 3 and 4are provided at the top of the furnace 1 for introducing two kinds ofmaterial gases. The furnace 1 sustains a Ga-boat 5 at an upper space. AGa-melt 6 is prepared by putting metal Ga into the Ga-boat 5 and heatingthe Ga-boat 5 by the heater 2. One gas inlet 3 has an open end facing tothe Ga-boat 5 for supplying H₂+HCl gas to the Ga-boat 5. The other gasinlet 4 has an open end at a middle height of the furnace forintroducing H₂+NH₃ gas.

A susceptor 7 is supported by a rotation shaft 8 in a lower half of thefurnace 1. The rotation shaft 8 can rotate, lift up or down thesusceptor 7. A GaAs substrate or a GaN substrate 9 is laid upon thesusceptor 7 as a seed. A GaN seed crystal can be prepared by making aGaN crystal on a GaAs substrate, eliminating the GaAs substrate andslicing the grown GaN crystal into wafers. The heater 2 heats thesusceptor 7 and the substrate 9. An intermediate compound galliumchloride GaCl gas is synthesized by blowing the HCl+H₂ gas to theGa-melt 6 in the boat 5. GaCl falls in the furnace near the substrate 9,reacts with ammonia and synthesizes gallium nitride (GaN) on thesubstrate 9. The HVPE uses a non-carbon material (Ga monoelement). TheHVPE is free from possibility of carbon contamination which degradeselectric properties of object crystals.

(4) Sublimation Method

Heating alone cannot convert solid GaN into a melt of Ga. High pressureis required for melting solid GaN besides heating. Difficulty of makinga GaN melt prohibits a Czochralski method or a Bridgman method fromgrowing a GaN solid from a GaN melt. Without high pressure, solid GaNsublimes into vapor GaN by heating. A sublimation method makes a GaNsingle crystal on a substrate by inserting a substrate and a GaNpolycrystal source into a reaction tube, heating the GaN polycrystalsource for subliming at a higher temperature, heating the substrate at alower temperature, transporting GaN vapor from the GaN source to thecolder substrate and depositing GaN molecules on the substrate.

Another improvement (Lateral Overgrowth Method) has been proposed formaking a low-dislocation density GaN film grown on a sapphire substratefor making on-sapphire GaInN-LEDs.

[Epitaxial Lateral Overgrowth Method (ELO)]

{circle around (1)} Akira Usui, “Thick Layer Growth of GaN by HydrideVapor Phase Epitaxy”, Electronic Information and Communication Society,C-II, vol. J81-C-II, No. 1, pp 58-64, (January, 1998), proposed a growthof GaN by a lateral overgrowth method. The lateral overgrowth methodproduces a low dislocation density GaN crystal by covering a sapphiresubstrate with a mask having dotted or striped windows lying at cornerpoints of periodically allocated equilateral triangles, supplyingmaterial gas via the mask windows to the sapphire substrate, depositingGaN molecules on the sapphire substrate within the windows, growingfurther GaN films from the windows over on the mask, joining neighboringGaN films in horizontal directions along boundaries between the windowson the mask, turning the growing direction from the horizontaldirections to the vertical direction and maintaining the vertical GaNgrowth on the mask. Dislocations have a tendency of extending along thegrowing direction. Many dislocations accompany GaN growth in any cases.The ELO method forces the dislocations to bend at the meeting boundariesabove the mask from horizontal extensions to an upward extension. Thechange of extension reduces the dislocations in the GaN crystal. Thus,the ELO is effective to reduce dislocation density of a GaN thin filmgrown on a sapphire substrate.

Inventors of the present invention chose the HVPE method as a verypromising candidate among the mentioned vapor phase growth methods forgrowing a thick GaN crystal for a freestanding GaN wafer. Almost all ofthe preceding trials for growing GaN films have started from sapphiresingle crystals as a substrate. Sapphire has, however, some drawbacks ofnon-existence of cleavage and impossibility of removal. The inventorsabandoned sapphire as a substrate for making a freestanding GaN singlecrystal.

Instead of sapphire, the inventors of the present invention chose GaAs(gallium arsenide) as a substrate for growing a thick GaN crystal formaking a freestanding GaN single crystal wafer. Although GaAs has acubic symmetry which is different from the hexagonal symmetry of GaN andthe trigonal symmetry of sapphire, a (111) GaAs plane has three-foldrotation symmetry akin to the hexagonal symmetry. The inventors of thepresent invention succeeded in growing a GaN crystal on a (111) GaAssubstrate in the c-direction from materials of metal gallium,hydrogen-diluted hydrochloric acid (HCl) gas and hydrogen-dilutedammonia gas. Fortunately, the GaAs substrate can be eliminated from thegrown GaN crystal by etching or polishing. Possibility of removal is anadvantage of GaAs as a substrate for making a freestanding GaN crystal.

{circle around (2)} Japanese Patent Application No.10-183446(183446/'98) was filed by the same inventor as the presentinvention. {circle around (2)} produced a GaN single crystal bypreparing a GaAs (111) substrate, covering the GaAs substrate with amask having windows, growing a thick GaN layer on the masked GaAssubstrate by the HVPE and the ELO method, and eliminating the GaAssubstrate by aqua regia. {circle around (2)} obtained a freestanding GaNbulk single crystal wafer having a 20 mm diameter and a 0.07 mmthickness. The GaN crystal was a C-(0001) plane crystal.

{circle around (3)} Japanese Patent Application No.10-171276(171276/'98) was filed by the same inventor as the presentinvention. {circle around (3)} also proposed a freestanding GaN bulksingle crystal wafer of a C-plane produced by depositing a thick GaNcrystal upon a (111) GaAs substrate. Distortion of the GaN wafer was aproblem. Distortion is induced on the freestanding GaN wafer bydifferences of thermal expansion between GaAs and GaN. How to reduce thedistortion was another problem for {circle around (3)}. Conduction typeof the GaN crystal was left untouched.

{circle around (4)} Kensaku Motoki, Takuji Okahisa, Naoki Matsumoto,Masato Matsushima, Hiroya Kimura, Hitoshi Kasai, Kikurou Takemoto, KojiUematsu, Tetsuya Hirano, Masahiro Nakayama, Seiji Nakahata, Masaki Ueno,Daijirou Hara, Yoshinao Kumagai, Akinori Koukitu and Hisashi Seki,“Preparation of Large Freestanding GaN Substrates by Hydride Vapor PhaseEpitaxy Using GaAs as a Starting Substrate”, Jpn. J. Appl. Phys. Vol. 40(2001) pp. L140-143, reported a freestanding GaN single crystal producedby a lateral overgrowth method upon a GaAs (111) substrate. Grown GaNwas a (0001) C-plane crystal having a 500 μm thickness and a 2 inchdiameter. The GaN crystal showed n-type conduction. Dislocation densitywas 2×10⁵ cm⁻². Carrier density was n=5×10¹⁸ cm⁻³. Mobility was 170cm²/Vs. Resistivity was 8.5×10−3 Ωcm. {circle around (4)} said nothingabout n-dopants.

{circle around (5)} Japanese Patent Application No. 11-144151 was filedby the same inventor as the present invention. {circle around (5)}proposed a freestanding n-type GaN single crystal containing oxygen asan n-dopant having nearly 100% of activation rate in GaN. This was thefirst document which asserted that oxygen was a good n-dopant in GaNwith nearly a 100% activation rate. Silicon (Si) has been prevailing asan n-dopant which has been exclusively doped into GaN thin films grownon sapphire substrates in a form of silane gas (SiH₄). But silane gas(SiH₄) is a dangerous gas. Oxygen can be supplied in a safe form ofwater or water vapor to material gases. {circle around (5)} rejectedsilicon but admitted oxygen as an n-dopant in GaN. {circle around (5)}insisted on replacement of silicon by oxygen. {circle around (5)}alleged that carbon (C) which is an n-impurity and disturbs the actionof oxygen should be excluded from the material gases. {circle around(5)} denied the MOCVD (metallorganic chemical vapor deposition) methodwhich uses metallorganic gases including plenty of carbon atoms butrecommended the HVPE (hydride vapor phase epitaxy) method.

GaN is a hexagonal symmetry crystal with three-fold rotational symmetry.Crystallographical representation of GaN is different from GaAs (zincblende type) which belongs to the cubic symmetry group. Crystallographicrepresentation of the hexagonal symmetry group is now described. Thereare two representations for hexagonal symmetry. One method uses threeparameters. The other method uses four parameters. Here, four parameterrepresentation which requires four axes is described. Three axes aredenoted by a-axis, b-axis and d-axis which lie on the same horizontalplane and meet at an origin with each other at 120 degrees. Unit lengthsa, b and d of the three axes are equal, that is, a=b=d.

An extra axis meets with other three axes at 90 degrees. The extra axisis named c-axis. A set of a-, b-, d-, and c-axes defines planes anddirections in a hexagonal symmetry crystal. The three a-, b-, d-axes areequivalent. But the c-axis is a unique axis. A set of plenty of parallelequivalent planes is imagined. When a first plane crosses a-axis at apoint of a/h, b-axis at a point of b/k, d-axis at a point of d/m andc-axis at a point of c/n, the plane is represented by (hkmn). When thefirst plane cannot cross a positive part of the axes, the axes should beextended in a negative direction for crossing with the first plane.Crystal has a periodic character. Thus, h, k, m and n are positive ornegative integers including zero (0). Number “h” means the number of theobject planes existing in a unit length “a” of a-axis. Number “k” meansthe number of the object planes existing in a unit length “b” of b-axis.The object plane is represented a round bracketed indices (hkmn).

Three equivalent indices h, k and m for a-, b-, d-axes always satisfy azero-sum rule of h+k+m=0. The other index n for c-axis is a freeparameter. Crystal indices h, k, m and n are substituted in a bracketwithout comma “,”. A negative index should be discriminated from apositive one by upperlining by the crystallograpy. Since an upperline isforbidden, a negative index is shown by adding a minus sign before theinteger. There are two index representations. One is an individualrepresentation. The other is a collective representation. Objects of theindex representation are planes and directions. A direction and a planetake the same set hkmn of indices, when the direction is a normal(meeting at 90 degrees) to the plane. But the kinds of brackets aredifferent.

Round bracketed (hkmn) means an individual representation of a plane.Wavy bracketed {hkmn} means a collective representation of familyplanes. Family planes are defined as a set of planes all of which can beconverted into other member planes by the symmetry operation included inthe crystal symmetry.

Besides the plane representation, linear directions should be denoted bya similar manner. Square bracketed [hkmn] means an individualrepresentation of a direction which is vertical to an individual plane(hkmn). Edged bracketed <hkmn> means a collective representation offamily directions. Family directions are defined as a set of directionsall of which can be converted into other member directions by thesymmetry operation of the crystal symmetry. The definitions are shown asfollows.

(hkmn) individual, plane. {hkmn} collective, plane. [hkmn] individual,direction. <hkmn> collective, direction.

In the hexagonal symmetry, C-plane is the most important planerepresented as (0001) which is normal to the horizontal plane includinga-, b- and c-axes. C-plane has three-fold rotational symmetry. All ofthe artificially made GaN crystals have been produced by C-plane growthwhich grows a crystal by maintaining C-plane as a surface. When GaN isheteroepitaxially grown on a foreign material, for example, sapphire(Al₂O₃) or gallium arsenide (GaAs), the seed surface should havethree-fold rotational symmetry. Thus, GaN grows on the foreign substratewith the three-fold rotation symmetry, maintaining C-plane which hasalso the same symmetry. Thus, heteroepitaxy on a foreign substrate isrestricted to C-plane growth. There are two secondary important planesnext to C-plane.

One important plane is {1-100} planes which are vertical to C-plane.This is a cleavage plane. The {1-100} planes mean a set of sixindividual planes (1-100), (10-10), (01-10), (−1100), (−1010) and(0-110) which are all cleavage planes. The (1-100), (10-10), (01-10),(−1100), (−1010) and (0-110) planes are called “M-plane” for short. Thecleavage planes meet with each other at 60 degrees. Any two cleavageplanes are not vertical.

The other important plane is {11-20} planes which are vertical toC-plane. The {11-20} planes mean a set of six individual planes (11-20),(1-210), (−2110), (2-1-10), (−12-10) and (−1-120). The (˜11-20),(1-210), (−2110), (2-1-10), (−12-10) and (−1-120) planes are called“A-plane” for short. A-planes are not cleavage planes. A-planes meetwith each other at 60 degrees.

C-plane {0001} is uniquely determined. But A-planes and M-planes are notuniquely determined, since A-planes and M-planes include three differentplanes. Some of A-planes are vertical to some of M-planes.

All A-planes are vertical to C-plane. All M-planes are vertical toC-plane. Some of A-planes, some of M-planes and C-plane can build a setof orthogonal planes.

{circle around (6)} Japanese Patent Application No.10-147049(147049/'98) was filed by the same inventor as the presentinvention. {circle around (6)} proposed non-rectangular GaN deviceswhich have sides of cleavage planes (M-planes). The GaN crystal hasC-plane as a surface.

{circle around (7)} Japanese Patent Application No.11-273882(273882/'99) was filed by the same inventor as the presentinvention. Conventional growth means a growth by maintaining amirror-flat, even C-plane surface. {circle around (7)} proposedfacet-growth of GaN along c-axis which keeps various facets on C-plane.The facets on C-plane signify small other planes than C-plane. Facetsform hexagonal pits or hillocks and dodecagonal pits or hillocks onC-plane. Although GaN grows on an average along the c-axis, variousfacets cover a surface of growing GaN. The facets sweep dislocationsdown to the bottoms of the facet pits. Dislocations are effectivelyreduced by the facets.

{circle around (8)} Japanese Patent Application No.2000-207783(207783/'00) was filed by the same inventor as the presentinvention. {circle around (8)} discovered a fact that dislocationsextend in parallel with the growing direction in a GaN crystal. C-planegrowth prolongs dislocations in parallel with the c-axis. {circle around(8)} proposed a sophisticated method of growing a tall GaN crystal inthe c-direction on a C-plane of a GaN seed, cutting the GaN crystal in Aplanes, obtaining an A-plane GaN seed crystal, growing the GaN crystalin an A-direction on the A-plane seed, cutting the A-grown GaN in Mplanes and obtaining M-plane GaN seeds with low dislocation density.Prior art of {circle around (1)} to {circle around (7)} grow C-plane GaNcrystals in the c-direction on a foreign material or a C-plane GaN seed.Only {circle around (8)} proposed non-C-plane growth of GaN on anon-C-plane GaN substrate.

All of the known attempts of on-sapphire GaN growth grow C-surface GaNcrystals having a C-surface as a top without exception. There is areason of the absolute prevalence of C-plane GaN crystals. When asapphire (α-Al₂O₃) single crystal substrate is used as a substrate, aC(0001) surface sapphire is used to be chosen. Sapphire belongs to atrigonal symmetry group which requires four indices for representingorientations of planes and directions. GaN has hexagonal symmetry.Lengths of c-axes are different between sapphire and gallium nitride.GaN has three typical, low index planes of A-plane, M-plane and C-plane,as explained before. A-plane or M-plane GaN cannot grow on A-plane orM-plane sapphire, because A-plane or M-plane are too complex to coincidewith a similar plane of sapphire without misfit. Only C-plane of GaN canjoin on C-plane of sapphire. Thus, a smooth, flat C-plane GaN crystalcan be easily grown on a C-plane sapphire substrate. All of the knownInGaN-LEDs have a pile of C-plane GaN or InGaN layers on a C-planesapphire substrate.

Similarly, in the case of GaAs substrates, a three-fold rotationallysymmetric (111) GaAs substrate is selected as a substrate. GaAs belongsto a cubic symmetry group. But a (111) plane of GaAs has three-foldrotational symmetry. The (111) GaAs substrate allows only C-plane GaNhaving the corresponding rotation symmetry to grow on. Foreign materialsas a substrate cannot grow non-C-plane GaN at all.

A thick bulk crystal requires far more amount of dopants than a thinfilm crystal. The amount of dopants is in proportion to a thickness orvolume of grown crystals. Silane gas (SiH₄) is a dangerous gas whichsometimes induces a burst. An n-type GaN bulk crystal would require alarger amount of an n-dopant than an n-type thin film. The inventorsprefer oxygen to silicon as the n-dopant, because water (H₂O) as anoxygen compound is far safer than silane gas (SiH₄). The inventors triedto dope GaN bulk crystals with oxygen as an n-dopant. However, mirrorflat, C-plane GaN crystals cannot be doped enough with oxygen. Theinventors discovered a fact that oxygen does not go into GaN easily viaC-plane and C-plane repulses oxygen. The inventors found orientationdependence of oxygen doping for the first time. The inventors were awareof the fact that oxygen doping has orientation dependence. The inventorsnoticed that C-plane is the worst plane for oxygen doping.

The orientation dependence of oxygen doping is a novel phenomenon. It isnot easy to understand the orientation dependence of oxygen doping.Nobody found the phenomenon before the inventors of the presentinvention. The inventors analysed atomic components on a surface ofC-plane grown GaN crystals by SIMS (Secondary Ion Mass Spectroscopy).The SIMS determines atomic ratios on a surface of an object sample byemitting first ions, accelerating the first ions, shooting the firstions at the sample for inducing secondary ions emitted out of thesample, analysing the mass of the secondary ions and counting thenumbers of the emitted secondary ions. The numbers of the emittedsecondary ions are proportional to products of emission efficiencies ofsecondary ions and atomic ratios on the object surface. Since theemission efficiencies are known parameters, the atomic ratios aredetermined.

At an early stage of the SIMS analysis, insufficient resolution of thesecondary ions and broadness of the first ion beam allowed a widesecondary beam to emanate from a wide area of the object. The broadsecondary ion beams obscured abnormality of oxygen doping. Then,C-planes of GaN seemed to emit oxygen secondary ions.

At a later stage, the inventors succeeded in converging the first ionbeam and enhancing resolution of the SIMS. Narrow converged first ionsand enhanced resolution revealed a surprising fact.

A C-plane surface of a C-grown GaN includes microscopic pits orhillocks. The microscopic hillocks and pits have many non-C-planes whichare called facets. A rough C-plane is an assembly of micro C-planes andmicro facets. Oxygen secondary ions were measured by discriminating themicro C-planes and micro non-C-plane facets. The inventors found thatthe oxygen secondary ions were emitted from the micro non-C-plane facetsand the micro C-planes do not emit the oxygen secondary ions. Namely,the micro C-planes included far smaller rate of oxygen than an averagerate. When oxygen concentration was 5×10¹⁸ cm⁻³ at non-C-plane facets,oxygen concentration was less than 1×10¹⁷ cm⁻³ at C-planes. The facetshave oxygen acceptance function which is more than 50 times as large asthat of C-planes. C-planes are the poorest in the function of acceptingoxygen. In the SIMS experiments, the secondary oxygen ions emanated notfrom C-planes but from the facets.

The inventors made a mirror-flat C-plane GaN crystal. The oxygenconcentration was less than 1×10¹⁷ cm⁻³ everywhere on a sample.

The SIMS analysis taught us that oxygen is hardly doped into C-plane ofGaN. The oxygen doping to a C-plane grown GaN crystal is caused by thefacets which have high acceptance function of oxygen.

When a GaN crystal grows in an average in the c-direction, oxygen can bedoped into GaN via the facets. Facets enable C-plane GaN to acceptoxygen as an n-dopant. The oxygen accepting power is in proportion tothe area of the facets. The wider the facets are, the stronger theoxygen acceptance power is. When C-plane is covered overall with facets,the oxygen acceptance power attains to the maximum.

Otherwise, an A-plane GaN seed and an M-plane GaN seed are bestsubstrates for growing a non-C-plane GaN crystal and doping thenon-C-plane GaN crystal with oxygen.

In conclusion, Si which is a prevalent n-dopant for thin films of GaN isnot suitable for doping large GaN bulk crystals. Oxygen is a safern-dopant. But conventional C-plane growth repulses oxygen. Oxygen dopingis insufficient for the C-plane growth. An A-plane GaN seed, an M-planeseed and facet c-axis growth are effective to oxygen doping into GaNcrystals.

SUMMARY OF THE INVENTION

The inventors have investigated possibility of oxygen doping to GaN bygrowing various orientations of GaN crystals. The inventors have foundan orientation (plane) dependence of the oxygen doping to GaN. Anaccumulation of experiments taught the inventors that a flat C-plane isan unfavorable orientation but all the orientations of planes are notunfavorable to the oxygen doping. The inventors noticed an existence oforientations of GaN crystals except C-plane which are favorable toaccept oxygen as an n-dopant. There are two types (1) and (2) ofoxygen-acceptable planes.

(1) {kk-2kh} planes (k, h; integer)

{11-20} planes, in particular, accept oxygen with a high rate. {11-22}planes also show a high doping efficiency of oxygen. Oxygen dopingefficiency is lower for higher numbers of indices of {kk-2kh}.

(2) {k-k0h} planes (k, h; integer)

{1-100} planes, in particular, are favorable planes for accepting oxygenwith a high rate. {1-101} planes also show a high doping efficiency ofoxygen. The higher the indices of {k-k0h} are, the lower the oxygendoping efficiency is.

Oxygen doping depends upon the orientation of planes. A {hkmn} plane hasan inherent power for doping GaN with oxygen. The inherent power ofdoping oxygen of the plane {hkmn} can be denoted by OD{hkmn}. Detaileddependence OD{hkmn} is not clear yet. C-plane takes the minimum of OD.Thus, for any planes other than C-plane, OD {hkmn}>OD {000}.

For special planes the oxygen doping power can be estimated by theresult of experiments.

A-planes {11-20} has more than 50 times as large power as C-plane;OD{11-20}>50OD{0001}.

M-planes {1-100} has more than 50 times as large power as C-plane;OD{1-100}>50OD{0001}.

Non-C-plane growth enables the growing GaN crystal to accept oxygeneffectively. Oxygen invades into the GaN crystal via non-C-planes on thesurface. One favorable case is to grow GaN having overall non-C-plane,for example, A-plane or M-plane. The other favorable case is to growfaceted GaN in the c-axis with various facets of non-C-planes on thesurface. Oxygen is absorbed via non-C-planes on the surface of growingGaN more effectively than C-plane. The orientation dependence of oxygendoping has been recently discovered by the inventors. The details arenot clear for the inventors yet. Coupling bonds appearing out of acrystal are changed by orientation of the surface. Elements to becoupled to the bonds are varied by the surface orientation. Thus,impurity doping may have dependence upon the orientation of the surface.

A (0001) Ga surface of GaN has high resistance against invasion ofoxygen to nitrogen sites as an n-impurity the inventors confirmed thatthe orientation dependence appears for all GaN growth upon sapphiresubstrates, silicon carbide substrates, gallium nitride substrates andso on.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of an HVPE (hydride vapor phase epitaxy)apparatus for growing a gallium nitride crystal.

FIG. 2 is sectional views of GaN crystals of steps of Embodiment 1 formaking an oxygen doped GaN single crystal in vapor phase on anM(1-100)-plane GaN single crystal seed. FIG. 2( a) is a section of theM(1-100)-plane GaN seed. FIG. 2( b) is a section of the M-plane GaN seedand a (1-100) GaN crystal homoepitaxially grown on the M(1-100)-planeGaN seed. FIG. 2( c) is a section of the grown (1-100) GaN crystal fromwhich the seed crystal has been removed by etching. FIG. 2( d) is asection of a polished grown (1-100) GaN crystal with an M-plane surface.

FIG. 3 is sectional views of GaN crystals of steps of Comparison Example1 for making a GaN single crystal in vapor phase on a C(0001)-plane GaNsingle crystal seed. FIG. 3( a) is a section of the C(0001)-plane GaNseed. FIG. 3( b) is a section of the C-plane GaN seed and a (0001) GaNcrystal homoepitaxially grown on the C(0001)-plane GaN seed. FIG. 3( c)is a section of the grown (0001) GaN crystal from which the seed crystalhas been removed by etching. FIG. 3( d) is a section of a polished grown(0001) GaN crystal with a C-plane surface.

FIG. 4 is sectional views of GaN crystals of steps of Embodiment 2 formaking an oxygen doped n-type GaN single crystal in vapor phase bymaintaining various non-C-plane facets on a C(0001)-plane GaN singlecrystal seed. FIG. 4( a) is a section of the C(0001)-plane GaN seed.FIG. 4( b) is a section of the C-plane GaN seed and a facet-growing(0001) GaN crystal grown on the C(0001)-plane GaN seed homoepitaxiallyin the c-axis <0001>. FIG. 4( c) is a section of the facet-growing(0001) GaN crystal from which the seed crystal has been removed byetching. FIG. 4( d) is a section of a polished grown (0001) GaN crystalwith a C-plane surface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The most effective method for doping gallium nitride crystal with oxygenis that water is added into material gases. In the case of an HVPEmethod, water is added into ammonia (NH₃) or hydrochloric (HCl) gases.When NH₃ and HCl inherently include water as an impurity, water needsnot be added into the material gases. In this case, the GaN crystal isconsequently doped with oxygen of water existing inherently in thegases. But, in order to dope with oxygen stably, it is desirable that afixed bit amount of water is added into the gases.

There are two alternative methods of doping a gallium nitride crystalsubstrate with oxygen effectively in accordance with the teaching of thepresent invention. One (A) is a non-C-plane method which grows galliumnitride upon a seed crystal having a non-C-plane surface. The other (B)is a C-plane facet-growing method which grows gallium nitride upon aseed crystal having a C-plane surface by maintaining manyvariously-oriented facets on the c-axis grown crystal upon the seedcrystal. The two are operative applications of the present invention.

(A) Non-C-Plane Method

Method (A) employs a gallium nitride seed crystal having a {hkmn}surface except C plane {0001}, grows a {hkmn} faced gallium nitridecrystal on the non-C-plane {hkmn} surface of the seed crystal andobtains a single crystal ingot extending in the same direction <hkmn> asthe {hkmn} surface seed crystal.

Method (A) starts from a non-C-plane {hkmn} oriented GaN single crystalseed, wherein {hkmn}≠{0001}. Method (A) maintains the non-C-plane {hkmn}on the surface of the growing crystal for injecting oxygen via thenon-C-plane to the growing non-C-plane crystal. Oxygen is effectivelysupplied overall into the growing non-C-plane gallium nitride crystal.

For example, a suitable candidate seed is a {1-100} plane (M-plane)gallium nitride crystal. In general, {k-k0h}-plane gallium nitridecrystals are candidates for the seeds for applying Method (A).

For example, another preferable candidate seed is a {11-20} plane(A-plane) gallium nitride crystal. In general, {kk-2kh}-plane galliumnitride crystals are candidates for the seeds for applying Method (A).

In Method (A), the oxygen doping efficiency OD can be expressed as afunction of the orientation indices simply by

-   -   OD=OD{hkmn}.

Method (A) is based upon a simple principle of selective oxygen dopingon the non-C-planes. Method (A), however, has still a problem forcarrying out. There is no natural gallium nitride single crystal havinga non-C-plane surface. Method (A) requires synthesis of a non-C-planegallium nitride single crystal as a seed by some means. No vapor phaseheteroepitaxial growth upon a foreign material substrate can produce anon-C-plane gallium nitride crystal. At present, all of the GaN or GaInNfilms which are heteroepitaxially produced upon sapphire substrates of aC-plane surface with three-fold rotational symmetry for making blueLEDs, are C-plane GaN or InGaN films. No non-C-plane GaN crystal can beheteroepitaxially made upon a sapphire substrate in any cases. Whenheteroepitaxy makes a GaN crystal upon a sapphire substrate, it isimpossible to obtain a freestanding isolated GaN crystal due to thedifficulty of eliminating the sapphire substrate from the GaN/sapphirecrystal. Sapphire is not suitable for a seed crystal of making afreestanding gallium nitride crystal having a non-C-plane surface.

Instead of sapphire, a gallium arsenide (111) crystal is a promisingcandidate for producing a gallium nitride crystal as a seed. A (111)gallium arsenide (GaAs) substrate enables vapor phase lateral overgrowthto make a gallium nitride (GaN) crystal growing along a c-axis. Unlikesturdy sapphire, the GaAs substrate can be eliminated from the GaNcrystal by aqua regia. Thus, a freestanding GaN single crystal can beobtained by heteroepitaxy upon a GaAs substrate. However, thefreestanding GaN crystal produced upon the (111) GaAs substrate is alsoa C-plane GaN single crystal having a C-plane surface. Thus, A-plane{11-20} surface GaN single crystals as seeds are obtained by growinganother tall GaN single crystal in the c-direction homoepitaxially onthe C-surface GaN single crystal and slicing the newly-grown GaN singlecrystal in {11-20} planes which are vertical to C-plane. The {11-20}surface crystals can be employed as a seed for growing a n-type GaNcrystal doped with oxygen. In general, {kk-2kh} plane crystals can bemade by heteroepitaxially growing a C-plane GaN single crystal on a(111) GaAs substrate, eliminating the GaAs substrate, homoepitaxiallygrowing a thick (tall) C (0001)-plane GaN single crystal on the C-planeGaN substrate and cutting the thick C-plane GaN in the {kk-2kh} plane.Thus, Method (A) requires double steps of growing GaN single crystalsfor preparing a seed.

(B) C-Plane Facet-Growing Method

Method (B) employs a C-plane gallium nitride seed crystal or a foreignmaterial crystal having three-rotational symmetry, grows the C-plane{0001} gallium nitride crystal having many facets of non-C-planes {hkmn}in vapor phase by supplying material gases including oxygen components,maintains the facets during the growth, obtains a C-plane faceted singlecrystal ingot extending in the c-axis direction <0001>, polishes thesurface of the facetted C-plane crystal, eliminates the superficialfacets from the surface of the GaN single crystal, and obtains an oxygendoped n-type C-plane GaN single crystal. The average plane of thegrowing crystal is the C-plane. But, the surface is covered with manyfacets having various planes {k-k0h}, {kk-2kh} and so on. The ratiooccupied by C-planes is small on the growing surface. Although C-planesreject oxygen, facets of various orientations effectively absorb oxygen.

In practice, favorable facet planes for allowing oxygen doping are{k-k0h} (k and h are integers) planes, in particular, {1-101} planes.These facets are obtained by inclining M {1-100} planes. Since M {1-100}planes are vertical to C {0001} plane, M-planes cannot be facets on C{0001} plane.

Other favorable facet planes for inducing oxygen doping are {kk-2kh} (kand h are integers) planes, in particular, {11-22} planes. These facetsare obtained by inclining A {11-20} planes. Since A {11-20} planes arevertical to C {0001} plane, A-planes cannot be facets on C {0001} plane.These are the case of including only a single kind of facets.

A GaN single crystal has six-fold rotational symmetry. A single kind offacets is a set of six different individual planes. For example, a facetrepresentation {kk-2kh} contains (kk-2kh), (kk-2k-h), (−2kkkh),(−2kkk-h), (k-2kkh) and (k-2kk-h). A single kind of facets can makehexagonal conical hills or hexagonal conical pits on a growing C-plane.Sometimes some of the six facets disappear. In the case, irregular pitsor hills of trigonal, rhombic, or pentagonal cones appear on the growingC-plane. The facets absorb oxygen.

Sometimes two kinds of facets coexist on the growing C-plane.Coexistence of various kinds of facets is more favorable for absorbingoxygen from material gases. In the present invention, oxygen doping isaccelerated by maintaining two kinds of facets {kk-2kh} and {k-koh} onthe growing C-plane. For example, six facets of {1-101} and six facetsof {11-22} can build regular dodecagonal conical pits or hillocks whichare effective to absorb oxygen. An assembly of more than two kinds offacets makes complex pits or hillocks which raise the efficiency ofoxygen doping. There is little fraction of C-planes on thefacet-covered, rugged GaN surface growing in the c-direction.

When two kinds of facets {kk-2kh} and {k-koh} accompany the c-directionGaN growth, hexagonal cone pits and dodecagonal cone pits enhance theprobability of absorbing oxygen. The growing GaN is converted into an ntype semiconductor by being doped with oxygen. The two kind facet methodhas a complex feature. The total efficiency OD of oxygen doping is a sumof the contributions from the different facets {hkmn}.

OD=ρ{hkmn}OD{hkmn}.

Here, ρ{hkmn} signifies the existence probability of the facet {hkmn} onthe faceted C-plane.

Oxygen doped facetted gallium nitride (GaN) single crystals are madeupon gallium arsenide (GaAs) substrates by all the methods of the HVPEmethod, the MOC method, the MOCVD method and the sublimation methodwhich have been employed for growing thin GaN films upon sapphire(Al₂O₃) substrates.

Embodiment 1 M(1-100) surfaced GaN crystal seed; FIG. 2

An M (1-100) plane GaN single crystal is prepared as a seed for puttingMethod (A) into practice by slicing a bulk GaN single crystal in theplanes parallel with an M (1-100) plane (FIG. 2( a)). The M-plane seedcrystal has a (1-100) top surface and a (−1100) bottom surface. The bulkGaN single crystal was obtained by growing a thick GaN single crystalupon a (111) GaAs substrate by a lateral overgrowth method and removingthe GaAs substrate by solving with aqua regia. The slicing direction M(1-100) is one of planes parallel with a growth direction <0001>. Thesliced M-surface crystal is polished. The prepared M-plane seed crystalhas a flat smooth surface which is immune from degraded superficiallayers.

An HVPE (hydride vapor phase epitaxy) method grows a gallium nitridecrystal on the M-plane GaN seed crystal under the following condition.

growth temperature 1020° C. NH₃ partial pressure 0.2 atm (2 × 10⁴ Pa)HCl partial pressure 1 × 10⁻² atm (10³ Pa) growth time 6 hours GaN layerthickness about 500 μm

The NH₃ material gas includes 2 ppm of water (H₂O). Water is added tothe NH₃ gas intentionally as an oxygen source. A (1-100) plane galliumnitride crystal of 500 μm in thickness is obtained, as shown in FIG. 2(b). The bottom seed GaN is removed away by polishing, as shown in FIG.2( c). The M-plane grown crystal is evenly polished. The polished GaNcrystal as shown in FIG. 2 (d) has a thickness of about 400 μm.

Electric properties of the grown M-plane-GaN crystal are investigated byHall's measurement. The Hall measurement confirms that carriers areelectrons. Average values of the Hall measurement at four points on theGaN crystal are,

carrier density=6×10¹⁸ cm⁻³

carrier mobility=160 Vs/cm².

The carrier density and mobility are uniform overall on the surface ofthe GaN crystal.

Element ratios existing at the surface of the grown GaN sample areanalysed by the SIMS (secondary ion mass spectroscopy). Theconcentrations of elements are,

hydrogen (H) 2 × 10¹⁷ cm⁻³ carbon (C) 3 × 10¹⁶ cm⁻³ oxygen (O) 8 × 10¹⁸cm⁻³ silicon (Si)  3 × 10¹⁷ cm⁻³.

Since the carriers are electrons and the carrier density (free electrondensity) is 6×10¹⁸ cm⁻³, the grown GaN is an n-type semiconductor.N-type impurities which have a probability of giving free electrons tothe GaN crystal are oxygen (O), carbon (C) and silicon (Si). Since ann-type dopant gives one electron to the matrix crystal, theconcentration of the n-type dopant should be higher than the carrierdensity. The carbon concentration (3×10¹⁶ cm⁻³) is lower than thecarrier density (6×10¹⁸ cm⁻³). The silicon concentration (3×10¹⁷ cm⁻³)is lower than the carrier density (6×10¹⁸ cm⁻³).

Only the oxygen concentration (8×10¹⁸ cm¹³) is higher than the carrierdensity (6×10¹⁸ cm⁻³). This fact means that the free carriers(electrons) derive from oxygen atoms, oxygen acts as an n-type dopantand oxygen has a high activation rate (about 0.8) in the GaN crystal.

Electrical conductivity is measured. The resistivity which is an inverseof conductivity is 7×10⁻³ Ωcm. The grown GaN crystal has a highconductivity of 140/Ωcm. The high conductivity enables the GaN singlecrystal to act as an n-type GaN substrate on which LDs or LEDs arefabricated. The known insulating sapphire substrates are suffering fromthe difficulty of non-cleavage and the difficulty of an n-electrode. Thegrown GaN substrate has good cleavage and good conduction which allows abottom n-electrode.

The grown GaN substrate of this embodiment is a freestanding GaNsubstrate having a flat, smooth surface and a thickness of 400 μm. Thisn-type GaN substrate can be used as a substrate of devices byepitaxially piling layers thereon.

Comparison Example 1 Flat C-Plane (0001) GaN Growth; FIG. 3

A C (0001) plane GaN single crystal is prepared as a seed by slicing abulk GaN single crystal in the planes parallel with a C (0001) plane(FIG. 3( a)) for making a comparison between C-plane growth and M-planegrowth. The C-plane seed crystal has a (0001) top surface and a (000-1)bottom surface. The top layer of the C-plane is a Ga layer, which issometimes represented by (0001)Ga. The bottom layer of the C-plane is anN layer, which is sometimes represented by (0001)N. The sliced C-surfacecrystal is polished. The prepared C-plane seed crystal has a flat smoothsurface which is immune from degraded superficial layers.

A flat, even, facetyless C(0001)-plane GaN crystal is grown on theC-plane GaN seed crystal by an HVPE (hydride vapor phase epitaxy) methodunder the following condition.

growth temperature 1050° C. NH₃ partial pressure 0.15 atm (1.5 × 10⁴ Pa)HCl partial pressure 5 × 10⁻³ atm (5 × 10² Pa) growth time 10 hours GaNlayer thickness about 500 μm

The NH₃ material gas includes 2 ppm of water (H₂O). Water is added tothe NH₃ gas intentionally as an oxygen source. The HVPE method maintainsa flat, even surface of the growing C-plane GaN crystal. A flat C(0001)plane gallium nitride crystal of 500 μm in thickness is obtained, asshown in FIG. 3( b). The top surface is a smooth, mirror (0001) plane.The bottom seed GaN is removed away by polishing, as shown in FIG. 3(c). The C-plane grown crystal is evenly polished. The polished GaNcrystal as shown in FIG. 3 (d) has a thickness of about 400 μm.

The Hall measurement for determining carrier density failed in the C—GaNComparison Example. The C-plane GaN specimen has too high resistivityand too low conductivity. The conductivity cannot be measured at anyspots upon the C—GaN specimen by the measurement tools available for theinventors. The C-plane GaN is an insulator which is poor in freecarriers. The C—GaN is neither n-type nor p-type but an intrinsic-typesemiconductor with high resistance. The SIMS (Secondary Ion MassSpectroscopy) measurement shows the ratios of elements on the topsurface of the C—GaN,

hydrogen (H) 1 × 10¹⁸ cm⁻³ carbon (C) 7 × 10¹⁶ cm⁻³ oxygen (O) 1 × 10¹⁷cm⁻³ silicon (Si) <2 × 10¹⁶ cm⁻³. 

Comparison Example 1 has far smaller oxygen concentration thanEmbodiment 1, although NH₃ gas includes the same rate (2 ppm) of water.The oxygen concentration (1×10¹⁷ cm⁻³) in Comparison Example 1 is aboutone hundredth ( 1/100) of Embodiment 1 (8×10¹⁸ cm⁻³). The carbonconcentration (7×10¹⁶ cm⁻³) is about twice as high as Embodiment 1(3×10¹⁶ cm⁻³). The silicon concentration (<2×10¹⁶ cm⁻³) is reduced downto one tenth of Embodiment 1 (3×10¹⁷ cm⁻³). The difference results fromthe difference of the growing surfaces (the M plane for Embodiment 1 andthe C plane for Comparison Example 1). The C-plane growth seems toenhance the absorption of carbon (C) and hydrogen (H). The C-planegrowth seems to suppress the doping of silicon (Si) and oxygen (O).Orientation dependence of silicon is smaller than that of oxygen. Oxygenexhibits the most conspicuous orientation dependence of doping.

Comparison Example 1 which is made by the flat C-plane growth cannotabsorb oxygen effectively. Poor oxygen intake incurs poor freeelectrons. Thus, the even C-plane growth GaN crystal becomes aninsulator. The insulating GaN crystal is not suitable for a substratefor making GaN devices, because an n-electrode cannot form on the bottomof the substrate.

Embodiment 2 Faceted C-Plane (0001) GaN Growth; FIG. 4

A C (0001) plane GaN single crystal is prepared as a seed by slicing abulk GaN single crystal in the planes parallel with a C (0001) plane(FIG. 4( a)). The C-plane seed crystal has a (0001) top surface and a(000-1) bottom surface. The top surface of the GaN seed is a (0001)Gaplane like Comparison Example 1. The sliced C-surface crystal ispolished. The prepared C-plane seed crystal has a flat smooth surfacewhich is immune from degraded superficial layers.

A faceted GaN crystal is grown in the c-direction on the C-plane GaNseed by an HVPE (hydride vapor phase epitaxy) method without eliminatingfacets under the following condition.

growth temperature 1030° C. NH₃ partial pressure 0.2 atm (2 × 10⁴ Pa)HCl partial pressure 1 × 10⁻² atm (10³ Pa) growth time 5 hours GaN layerthickness about 500 μm

The NH₃ material gas includes 2 ppm of water (H₂O). Water is added tothe NH₃ gas intentionally as an oxygen source. The HVPE method maintainsfaceted surface containing various pits or hillocks on the growingC-plane GaN crystal. A lower temperature of 1030° C., a higher NH₃partial pressure of 0.2 atm and a higher HCl partial pressure of 1×10⁻²atm enable a C-plane GaN crystal to maintain facet-growth. A ruggedC(0001) plane gallium nitride crystal of 500 μm in thickness isobtained, as shown in FIG. 4( b). The top surface is a rugged (0001)plane covered with many facets of various orientations except a (0001)plane. Almost all of the top is occupied by the facets. The grown GaN isglittering by the facets. Plenty of hexagonal conical pits anddodecagonal conical pits are found on the GaN crystal. The ratio ofC-planes is very small on the top surface.

The facets on the top include various orientations. Prevalent facets are{1-101} planes, {11-22} planes, {1-102} planes, {11-24} planes and soon. These facets can be represented collectively by {k-k0h} planes (k,h: integer) or {kk-2kh} planes (k, h: integer).

The bottom (0001) seed GaN is removed away by polishing, as shown inFIG. 4( c). The average thickness is about 400 μm. The top of thefacet-grown crystal is evenly polished for removing the facets. Thepolished GaN (0001) crystal as shown in FIG. 4 (d) has a thickness ofabout 350 μm.

Electrical properties are investigated by Hall measurement at four spotsof the GaN crystal of Embodiment 2. Averages at the four points are,

carrier density=5×10¹⁸ cm⁻¹

carrier mobility=170 Vs/cm².

The SIMS (Secondary Ion Mass Spectroscopy) measurement shows the ratiosof elements on the top surface of the C—GaN,

hydrogen (H) 2 × 10¹⁷ cm⁻³ carbon (C) 3 × 10¹⁶ cm⁻³ oxygen (O) 5 × 10¹⁸cm⁻³ silicon (Si) <4 × 10¹⁶ cm⁻³. 

Embodiment 2 has 5×10¹⁸ cm⁻¹ oxygen concentration and 5×10¹⁸ cm⁻³carrier concentration. The oxygen concentration is equal to the carrierconcentration. The oxygen concentration (5×10^(18 cm) ⁻³ of Embodiment 2is 50 times as much as Comparison Example 1 (1×10¹⁷ cm⁻³). Si, C and Ohave a probability of acting an n-dopant in GaN. But the siliconconcentration (<4×10¹⁶ cm⁻³) and the carbon concentration (3×10¹⁶ cm⁻³)are far smaller than the carrier concentration (5×10^(18 cm) ⁻³) Thefact confirms that free electrons of 5×10¹⁸ cm⁻³ are generated by oxygenatoms as an n-dopant with a high activation rate nearly equal to 100%.

The difference of oxygen concentration between Comparison Example 1 andEmbodiment 2 results from the difference of the facet-growth or thenon-facet, mirror flat growth in the c-direction. The flat C-planegrowth seems to suppress the doping of oxygen. The faceted C-planegrowth allows microscopic facets to absorb oxygen atoms effectively. Themany microscopic {kk-2kh} facets or {k-k0h} facets can absorb oxygenatoms like the allover M-plane of Embodiment 1. Orientation dependenceof oxygen-doping is strong. C-planes are inherently the poorest planefor doping with oxygen. But, the faceted C-plane growth can dope a GaNcrystal with oxygen at a high rate, because rugged C-plane includes manyfacets which accelerate oxygen doping.

Electrical conductivity is measured. The resistivity which is an inverseof conductivity is 6×10⁻³ Ωcm. The faceted C-grown GaN crystal has ahigh conductivity of about 170/Ωcm. The high conductivity enables theGaN single crystal to act as an n-type GaN substrate on which LDs orLEDs are fabricated. The known insulating sapphire substrates aresuffering from the difficulty of non-cleavage and the difficulty of ann-electrode. The present invention gives a Si-undoped, n-type GaN bulksingle crystal substrate which is doped with oxygen and has goodcleavage and good conduction which allows a bottom n-electrode.

The faceted C-grown GaN substrate of Embodiment 2 is a freestanding GaNsubstrate having a flat, smooth surface and a thickness of 350 μm afterthe facets have been removed by polishing. The obtained n-type GaNsubstrate is available for a substrate of devices by epitaxially pilinglayers thereon.

1-20. (canceled)
 21. An oxygen doping method to a gallium nitride singlecrystal substrate comprising the steps of: preparing a C-plane galliumnitride single crystal seed having a C-plane surface or a foreignmaterial single crystal seed having three-rotational symmetry; supplyingthe C-plane gallium nitride single crystal seed or the foreign materialsingle crystal seed with material gases including a gallium material, anitrogen material and an oxygen containing material; growing a galliumnitride bulk crystal that has micro C-planes and micro hexagonal ordodecagonal cone pits consisting of micro facets having non-C-planesupon the C-plane gallium nitride single crystal seed or the foreignmaterial single crystal seed in vapor phase; maintaining the microC-planes and the micro facets having the non-C-planes on the growinggallium nitride bulk crystal; doping the growing gallium nitride bulkcrystal with oxygen via the micro facets with an efficiency more than 50times as large as via the micro C-planes; eliminating the foreignmaterial single crystal seed or the C-plane gallium nitride singlecrystal seed from the grown gallium nitride bulk crystal by etching orpolishing; polishing the faceted surface of the grown gallium nitridebulk crystal for eliminating the facets; and obtaining a flat C-planesurface gallium nitride substrate having an n-type carrier density ofmore than or equal to 5×10¹⁸ cm⁻³ at points which were covered with themicro facets.
 22. The method as claimed in claim 21, wherein the microhexagonal cone pit consists of a set of six facets of {kk-2kh} planes(k,h; integer) or a set of six facets of {k-k0h} planes.
 23. The methodas claimed in claim 21, wherein the micro dodecagonal cone pit consistsof a set of six facets of {kk-2kh} planes and six facets of {k-k0h}planes (k,h; integer).
 24. A method of making gallium nitride devicescomprising the steps of: preparing a C-plane gallium nitride singlecrystal seed having a C-plane surface or a foreign material singlecrystal seed having three-rotational symmetry; supplying the C-planegallium nitride single crystal seed or the foreign material singlecrystal seed with material gases including a gallium material, anitrogen material and an oxygen containing material; growing a galliumnitride bulk crystal that has micro C-planes and micro hexagonal ordodecagonal cone pits consisting of micro facets having non-C-planesupon the C-plane gallium nitride single crystal seed or the foreignmaterial single crystal seed in vapor phase; maintaining the microC-planes and the micro facets having the non-C-planes on the growinggallium nitride bulk crystal; doping the growing gallium nitride bulkcrystal with oxygen via the micro facets with an efficiency more than 50times as large as via the micro C-planes; eliminating the foreignmaterial single crystal seed or the C-plane gallium nitride singlecrystal seed from the grown gallium nitride bulk crystal by etching orpolishing; polishing the faceted surface of the grown gallium nitridebulk crystal for eliminating the facets; obtaining a flat C-planesurface gallium nitride substrate with a top and a bottom having ann-type carrier density of more than equal to 5×10¹⁸ cm⁻³; and depositingan n-electrode on the bottom of the C-plane surface gallium nitridesubstrate.
 25. An oxygen doped gallium nitride single crystal with ahydrogen concentration of less than or equal to 2×10¹⁷ cm⁻³ and having aC-plane surface, produced by the steps of: preparing a C-plane galliumnitride single crystal seed having a C-plane surface or a foreignmaterial single having three-rotational symmetry; supplying the C-planegallium nitride single crystal seed or the foreign material singlecrystal seed with material gases including a gallium material, anitrogen material and an oxygen containing material; growing a galliumnitride bulk crystal that has micro C-planes and micro hexagonal ordodecagonal cone pits consisting of micro facets having non-C-planesupon the C-plane gallium nitride single crystal seed or the foreignmaterial single crystal seed in vapor phase; maintaining the microC-planes and the micro facets having the non-C-planes on the growinggallium nitride bulk crystal; doping the growing gallium nitride bulkcrystal with oxygen via the micro facets with an efficiency more than 50times as large as via the micro C-planes; eliminating the foreignmaterial single crystal seed or the C-plane gallium nitride singlecrystal seed from the grown gallium nitride bulk crystal by etching orpolishing; polishing the faceted surface of the grown gallium nitridebulk crystal for eliminating the facets; and obtaining a flat C-planesurface gallium nitride substrate having an n-type carrier density ofmore than or equal to 5×10¹⁸ cm⁻³ at points which were covered with themicro facets and having an n-type carrier density of less than or equalto 1×10¹⁷ cm⁻³ at points which were covered with the C-planes.
 26. Theoxygen doped gallium nitride single crystal substrate as claimed inclaim 25, wherein the micro hexagonal cone pit consists of a set of sixfacets of {kk-2kh} planes (k,h; integer) or a set of six facets of{k-k0h} planes.
 27. The oxygen doped gallium nitride single crystalsubstrate as claimed in claim 25, wherein the micro dodecagonal cone pitconsists of a set of six facets of {kk-2kh} planes and six facets of{k-k0h} planes (k,h; integer).
 28. A nitride device produced by thesteps of; preparing a C-plane gallium nitride single crystal seed havinga C-plane surface or a foreign material single crystal seed havingthree-rotational symmetry; supplying the C-plane gallium nitride singlecrystal seed or the foreign material single crystal seed with materialgases including a gallium material, a nitrogen material and an oxygencontaining material; growing a gallium nitride bulk crystal with microC-planes and micro hexagonal or dodecagonal cone pits consisting ofmicro facets having non-C-planes upon the C-plane gallium nitride singlecrystal seed or the foreign material crystal seed in vapor phase;maintaining the micro C-planes and the micro facets having thenon-C-planes on the growing gallium nitride bulk crystal; doping thegrowing gallium nitride bulk crystal with oxygen via the micro facetswith an efficiency more than 50 times as large as via the microC-planes; eliminating the foreign material single crystal seed or theC-plane gallium nitride single crystal seed from the grown galliumnitride bulk crystal by etching or polishing; polishing the facetedsurface of the grown gallium nitride bulk crystal for eliminating thefacets; obtaining a flat C-plane surface gallium nitride substrate witha top and a bottom having an n-type carrier density of more than orequal to 5×10¹⁸ cm⁻³; depositing an n-electrode on the bottom of theflat C-plane surface gallium nitride substrate; piling layers on the topof the C-plane surface gallium nitride substrate; and dividing thelayers piled gallium nitride substrate into device chips.